Subway

A Subway is the North American term for a rapid transit system that runs underground for much or all of its route. Outside North America, they may be known as an underground, metro, or mass transit railway. Subways require large populations to run efficiently and run at a "grade separation" from other vehicles and run frequently. This means that they exist in the largest cities.

The first subway was opened in London in 1863, (where it is called the "Underground") although many more major cities now have subways systems. The Boston subway opened in 1901 and the New York subway first opened in 1904.